Transponder

ABSTRACT

Proposed is a transponder for receiving a wireless electromagnetic interrogation signal and for transmitting a wireless electromagnetic response signal with a first coil acting as an antenna for generating a first wired electrical receive signal from the interrogation signal, and with at least one further coil acting as an antenna for generating a further wired electrical receive signal from the interrogation signal, wherein an axis of the first coil and an axis of the further coil are aligned differently in the space, wherein in each case one full-wave rectifier for rectifying the respective receive signal is assigned to the coils, wherein a summing element for summing up the rectified receive signals generated by the full-wave rectifiers is provided in order to generate in this manner a pulsating sum signal, the frequency of which corresponds to twice the frequency of the interrogation signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of previously filed copendingapplication Ser. No. 13/464,244, filed May 4, 2012, which claimspriority under 35 U.S.C. §119 to German Application No. 102011050129.0,filed May 5, 2011, which applications are hereby incorporated herein byreference in their entireties and from which applications priority ishereby claimed.

BACKGROUND

A. Technical Field

The present invention relates to a transponder for receiving a wirelesselectromagnetic interrogation signal and for transmitting a wirelesselectromagnetic response signal with a first coil acting as an antennafor generating a first wired electrical receive signal from theinterrogation signal, and with at least one further coil acting as anantenna for generating a further wired electrical receive signal fromthe interrogation signal, wherein an axis of the first coil and an axisof the further coil are aligned differently in the space.

B. Background of the Invention

A transponder having three coils which are each arranged orthogonally toeach other is known from EP0783190A1. Here, the signals of the threecoils are in each case rectified by means of a diode and are used forcharging a capacitor arrangement. The direct current voltage applied tothe capacitor arrangement is then used as supply voltage for thetransponder. By using the three coils arranged in each case orthogonallyto each other, the supply voltage can be provided substantiallyindependent of the transponder's position and spatial orientation withregard to the interrogator generating the interrogation signal.

However, in this manner, it is not possible to generate from theinterrogation signal a time base signal which corresponds with thefrequency of the interrogation signal and which may be required forsynchronizing the transponder with the interrogation device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a transponder whichis configured for generating a time base signal from an interrogationsignal, wherein generating the time base signal shall be substantiallyindependent of the transponder's position and spatial orientation withregard to the interrogator generating the interrogation signal.

The object is achieved with a transponder of the aforementioned kind inthat in each case one full-wave rectifier for rectifying the respectivereceive signal is assigned to the coils, wherein a summing element forsumming up the rectified receive signals generated by the full-waverectifier is provided in order to generate in this manner a pulsatingsum signal, the frequency of which corresponds to twice the frequency ofthe interrogation signal.

A transponder is to be understood as a transmitting and receiving unitwhich upon reception of a wireless electromagnetic interrogation signal,transmits a wireless electromagnetic response signal. The interrogationsignal is generated by an interrogator which is specifically configuredfor this and usually is also configured for receiving the responsesignal. Such combinations of transponders and interrogators can inparticular be used for keyless access control and/or keyless usagecontrol of motor vehicles.

Coils, in particular cylindrical coils, can generally be used asantennas for receiving wireless electromagnetic signals, wherein theysubstantially respond to the magnetic component of the field and convertit into a wired electrical signal. However, for this, they have adistinct directivity. If the axis of the coil is oriented toward thetransmitter, the result is a reception minimum; in contrast, if the axisis perpendicular to the direction toward the transmitter, the result isa reception maximum.

Since the transponder according to the invention has at least two coils,the axes of which are aligned differently in the space, thus are notparallel, it is ensured that independent of the transponder's positionand spatial orientation with regard to the interrogator generating theinterrogation signal, at least one coil operates outside of thereception minimum. In this manner, at least one wired electrical receivesignal is generated from the interrogation signal, wherein said receivesignal has a level which is well above the level at the receptionminimum.

A full-wave rectifier is generally to be understood as a rectifier inwhich both half-waves of the fed signal are provided with the same signso that a pulsating DC signal is generated which has double thefrequency of the fed signal. The full-wave rectifier can be a bridgerectifier which is formed by four diodes arranged in a full-wave bridgecircuit (Graetz rectifier). According to the invention, each coil isprovided with a full-wave rectifier so that from each receive signal, arectified receive signal is obtained.

The rectified receive signals are then summed up by means of a summingelement so that a pulsating sum signal is generated, the frequency ofwhich corresponds to twice the frequency of the interrogation signal.The full-wave rectification ensures that the absolute values of thereceive signals are added up regardless of their sign. In this manner itis ensured that the receive signals do not weaken each other or evencancel each other out during the adding process, which would happenwithout rectification if the transponder is aligned such that one of thecoils is penetrated by the interrogation signal in the one direction,and another one of the receiving coils is penetrated in the otherdirection. In this case, the (non-rectified) receive signal of the onecoil and the (non-rectified) receive signal of the other coil would havea phase shift of 180° so that during adding up, the two receive signalswould partially, in extreme cases even completely, cancel each otherout. The complete cancellation would occur when the levels of the tworeceive signal have the same value.

In contrast to that, the transponder according to the invention useseach half-wave of each of the receive signals for increasing the levelof the sum signal so that independent of the transponder's position andspatial orientation with regard to the interrogator generating theinterrogation signal, a stable sum signal is generated. The frequency ofthe sum signal corresponds here to twice the frequency of the receivedinterrogation signal so that it can be used as a time base signal inparticular for synchronizing the transponder with the interrogator.

The transponder according to the invention allows in particular thegeneration of a time base signal from an amplitude-modulatedinterrogation signal, the amplitude of which is changed in order totransmit different values, since the sum signal can be reliablygenerated even if the amplitude of the interrogation signal is verysmall due to the modulation.

According to an advantageous refinement of the invention, a total ofthree coils are provided, the axes of which are in each case arrangedperpendicular to each other. In this manner, a coil arrangement iscreated which, as a whole, has only a very low directivity so that thesum signal can be reliably formed at any spatial orientation of thetransponder.

According to an advantageous refinement of the invention, a comparatoris provided for comparing the pulsating sum signal with a referencesignal in order to generate in this manner a first clock signal, thefrequency of which corresponds to twice the frequency of theinterrogation signal. In general, a comparator is a circuit forcomparing two input signals. Depending on which of the two input signalsis greater, the output signal of the comparator takes a first value or asecond value. Comparing now the sum signal by means of the comparatorwith a reference signal results in a binary clock signal at the outputof the comparator, the frequency of which binary clock signalcorresponds to twice the frequency of the interrogation signal, whereinthe binary clock signal can be further processed in a simple manner. Thecomparator can be formed by means of a conventional operationalamplifier.

According to an advantageous refinement of the invention, a low pass isprovided for filtering the pulsating sum signal, wherein a filteredpulsating sum signal generated by means of the low pass is the referencesignal fed to the comparator. A low pass is generally a filter whichallows signal portions below a cutoff frequency to pass almost withoutattenuation and increasingly attenuates signal portions above the cutofffrequency. If now by means of such a low pass, a filtered pulsating sumsignal is generated from the sum signal and used as a reference signal,an automatic adaptation of the reference signal to the level of the sumsignal takes place so that changes of the current value of the sumsignal result in a change of the value of the first binary clock signalso that the first clock signal comprises the clock informationindependent of the level of the sum signal.

According to an advantageous refinement of the invention, the firstclock signal is fed to a frequency divider so as to generate a secondclock signal with a lower frequency. A frequency divider is generally anarrangement which reduces the frequency of an input signal by a factor,preferably by an integer factor. Using a frequency divider allows togenerate the second clock signal with a frequency as needed, whereinalso the second clock signal contains the information on which frequencyand phase that the interrogation signal has.

According to an advantageous refinement of the invention, the frequencydivider is a frequency halver so that the second clock signal has thefrequency of the interrogation signal. In the case of a frequencyhalver, the output signal generally has a frequency which corresponds tohalf of the frequency of the input signal. Thus, in the present case, asecond clock signal is generated in a simple manner, the frequency ofwhich corresponds to the frequency of the interrogation signal.

According to an advantageous refinement of the invention, each of thecoils is associated with a limiter for limiting the amplitude of therespective receive signal. A limiter is to be understood as anarrangement which processes an input signal in such a manner that theoutput signal of the limiter does not exceed a maximum value. In thepresent case, the limiters can be configured for limiting the voltage ofthe respective receive signal in order to protect downstream componentsagainst high voltages which can be generated when receiving stronginterrogation signals and/or interference signals. The limiters canadvantageously be connected in a directly conductive manner to therespective coil, thus without interposing further components, in orderto protect all components connected downstream of the coil.

According to a preferred refinement of the invention, in each case oneamplifier for amplifying the respective receive signal is assigned tothe coils. In this manner, the receive signals can be brought to a levelsufficient for further processing, in particular for full-waverectifying, even if the receive signal is very weak. The amplifiers caninvolve in particular voltage amplifiers. The amplifiers are preferablyconnected directly downstream of the limiter.

According to a preferred refinement of the invention, the limiter andthe amplifier are configured as a structural unit. In this manner, thestructure of the transponder is simplified.

According to a preferred refinement of the invention, in each case onevoltage-to-current converter is assigned to the coils in order to feedthe respective receive signal as a current signal to the summingelement. A voltage-to-current converter is generally to be understood asa voltage-controlled current source. In the present case, the receivesignal can be converted into a current signal which makes summing up thereceive signals easier. In particular, the summing element can beconfigured as feedback operational amplifier, wherein the receivesignals are fed together directly to the inverted input, thus withoutohmic resistors connected upstream, which simplifies the structure ofthe transponder. Expediently, the voltage-to-current converters can bearranged in each case between the associated full-wave rectifier and theassociated amplifier.

According to an advantageous refinement of the invention, thevoltage-to-current converters are operational transconductanceamplifiers. Operational transconductance amplifiers (abbreviated: OTA)are special operational amplifiers which have a high-impedance currentoutput. By means of such operational transconductance amplifiers, thereceive signals can be converted in a particularly simple manner intocurrent signals.

According to an advantageous refinement of the invention, thetransponder is at least partially implemented as a CMOS-integratedcircuit. An integrated circuit is an integrated circuitry, which meansthat an electronic circuit comprising a plurality of electroniccomponents and associated wiring is formed on a common substrate, alsocalled chip. For this, a fully integrated construction can be provided,in which all electronic components of the transponder are arranged onexactly one substrate.

Furthermore, the transponder according to the invention can be producedusing CMOS technology, which means that PMOS transistors, also calledp-channel metal-oxide-semiconductor transistors, as well as NMOStransistors, also called n-channel metal-oxide-semiconductortransistors, can be arranged on a common substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its refinements is explained in more detail by meansof the following figures. In the figures:

FIG. 1 shows an exemplary embodiment of a transponder according to theinvention and an associated interrogator in a schematic illustration,and

FIG. 2 shows a circuit diagram of the transponder of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a transponder system 1 that consists of an interrogator 2and a transponder 3. Such a combination 1 of interrogator 2 andtransponder 3 can in particular be used for keyless access controland/or for keyless usage control of motor vehicles. However, otherapplications are also possible.

The basic function is that a wireless interrogation signal AS with afrequency f0 is emitted by the interrogator 2. If only one transponder 3is in range, said transponder receives the interrogation signal AS,evaluates it and generates a wireless response signal which is emittedby the transponder 3 and received and evaluated by the interrogator.

For this, the interrogator 2 has at least one coil L1 which acts as anantenna and has the connections P+ and P−. The transponder 3 has afirst, in particular cylindrical, coil L1 which acts as an antenna andhas an axis A1 and connections S1+ and S1−, a second, in particularcylindrical, coil L2 which acts as an antenna and has an axis A2 andconnections S2+ and S2−, and a third, in particular cylindrical, coil L3which acts as an antenna and has an axis A3 and connections S3+ and S3−.

The coils L1, L2, L3 each individually have a distinct directivity. Ifthe respective axis A1, A2, A3 of a coil L1, L2, L3 points toward theinterrogator 2, a reception minimum occurs; in contrast, if said axis isperpendicular to the direction toward the interrogator 2, a receptionmaximum occurs.

Since the axes A1, A2, A3 of the coils L1, L2, L3 are arrangedperpendicular to each other, it is ensured, independent of thetransponder's 3 position and spatial alignment, that not more than oneof the coils L1, L2, L3 is operated at a reception minimum and that thetwo remaining coils of the coils L1, L2, L3 are operated under favorableconditions. In this manner, interrogation signals AS can be receivedsubstantially independently of the transponder's 3 position and spatialorientation with regard to the interrogator 2 generating theinterrogation signal AS.

FIG. 2 shows a circuit diagram of the transponder 1 of the FIG. 1. Here,only that portion of the circuit of the transponder 1 is shown which isessential for understanding the invention. The coils L1, L2, L3 are usedas antennas for receiving the interrogation signal AS, wherein theyrespond substantially to the magnetic component of the interrogationsignal AS and convert it in each case in a wired, electrical receivesignal E1, E2, E3.

The receive signals E1, E2, E3 are fed in each case to a structural unit4.1, 4.2, 4.3 which has on its input side a limiter 4.1, 4.2, 4.3 and anamplifier 4.1, 4.2, 4.3 connected downstream of the latter.

The limiters 4.1, 4.2, 4.3 are configured for limiting the voltage ofthe respective receive signal E1, E2, E3 in order to protect in thismanner downstream components against high voltages which can occur whenreceiving strong interrogation signals and/or interference signals. Thelimiters 4.1, 4.2, 4.3 are advantageously connected in a directlyconductive manner to the respective coil L1, L2, L3, thus withoutinterposing further components, in order to protect all componentsconnected downstream of the respective coil L1, L2, L3. The amplifiers4.1, 4.2, 4.3 are connected directly downstream of the respectivelimiter 4.1, 4.2, 4.3 and are implemented as voltage amplifiers. In thismanner, the receive signals E1, E2, E3 can be brought to a levelsufficient for further processing, even if the receive signal is veryweak.

The limited and amplified receive signals E1′, E2′, E3′ are now fed ineach case to a voltage-to-current converter 5.1, 5.2, 5.3 in order toconvert the respective receive signal E1′, E2′, E3′ into a currentsignal. The voltage-to-current converters 5.1, 5.2, 5.3 can preferablybe configured as operational transconductance amplifiers 5.1, 5.2, 5.3.

The receive signals E1″, E2″, E3″ now present as current signals are fedin each case to a full-wave rectifier 6.1, 6.2, 6.3. A full-waverectifier 6.1, 6.2, 6.3 is generally to be understood as a rectifier6.1, 6.2, 6.3 in which both half-waves of the fed signal E1″, E2″, E3″are provided with the same sign so that a pulsating DC signal GE1, GE2,GE3 is generated which has double the frequency of the fed signal E1″,E2″, E3″. The respective full-wave rectifier 6.1, 6.2, 6.3 can be abridge rectifier which is formed by four diodes arranged in a full-wavebridge circuit (Graetz rectifier). According to the invention, each coilL1, L2, L3 is provided with a full-wave rectifier 6.1, 6.2, 6.3 so thatfrom each receive signal E1″, E2″, E3″, a rectified receive signal GE1,GE2, GE3 is obtained.

The rectified receive signals GE1, GE2, GE3 are then summed up by meansof a summing element 7 so that a pulsating sum signal SG is generated,the frequency of which correspond to twice the frequency of theinterrogation signal AS. The full-wave rectification ensures that theabsolute values of the receive signals E1″, E2″, E3″ are added up,regardless of their sign. In this manner, it is ensured that the receivesignals E1″, E2″, E3″ do not weaken each other or even cancel each otherout during the adding process, which would happen without rectificationif the transponder is aligned such that one of the receiving coils L1,L2, L3 is penetrated by the interrogation signal AS in the onedirection, and another one of the receiving coils L1, L2, L3 ispenetrated in the other direction. In this case, the (non-rectified)receive signal E1″, E2″, E3″ of the one coil L1, L2, L3 and the(non-rectified) receive signal E1″, E2″, E3″ of the other coil L1, L2,L3 would have a phase shift of 180° so that during adding up, the tworeceive signals E1″, E2″, E3″ would partially, in extreme cases evencompletely, cancel each other out. The complete cancellation would occurwhen the levels of the two receive signal E1″, E2″, E3″ have the samevalue.

In contrast to that, the transponder 3 according to the invention useseach half-wave of each of the receive signals E1″, E2″, E3″ forincreasing the level of the sum signal SG so that independent of thetransponder's 3 position and spatial orientation with regard to theinterrogator 2 generating the interrogation signal AS, a stable sumsignal SG is generated. The frequency of the sum signal SG correspondshere to twice the frequency of the received interrogation signal AS sothat it can be used as a time base signal in particular forsynchronizing the transponder 3 with the interrogator 2.

The sum signal SG is fed to a comparator 8 which compares the pulsatingsum signals SG with a reference signal RS in order to generate a firstclock signal TS1, the frequency of which corresponds to twice thefrequency of the interrogation signal AS. In general, a comparator 8 isa circuit for comparing two input signals SG, GS. Depending on which ofthe two input signals SG, GS is greater, the output signal TS1 of thecomparator 8 takes a first value or a second value. Comparing now thesum signal SG by means of the comparator 8 with a reference signal RSresults in a binary clock signal TS1 at the output of the comparator 8,the frequency of which binary clock signal corresponds to twice thefrequency of the interrogation signal AS, wherein the binary clocksignal TS1 can be further processed in a simple manner. The comparator 8can be formed by means of a conventional operational amplifier.

Here, a low pass 9 is provided for filtering the pulsating sum signalSG, wherein a filtered pulsating sum signal SG generated by means of thelow pass 9 is the reference signal RS fed to the comparator 8. A lowpass 9 is generally a filter which allows signal portions below a cutofffrequency to pass almost without attenuation and increasingly attenuatessignal portions above the cutoff frequency. If now by means of such alow pass 9, a filtered pulsating sum signal RS is generated from the sumsignal SG and used as a reference signal RS, an automatic adaptation ofthe reference signal RS to the level of the sum signal SG takes place sothat changes of the current value of the sum signal SG result in achange of the value of the first binary clock signal T 1 so that thefirst clock signal TS1 comprises the clock information independent ofthe level of the sum signal SG.

Furthermore, the first clock signal TS1 is fed to a frequency divider 10so as to generate a second clock signal TS2 with a lower frequency. Afrequency divider 10 is generally an arrangement which reduces thefrequency of an input signal TS1 by a factor, preferably by an integerfactor. Using a frequency divider 10 allows generating the second clocksignal TS2 with a frequency as needed, wherein also the second clocksignal TS2 contains the information on which frequency that theinterrogation signal has.

Preferably, the frequency divider 10 is a frequency halver 10. In thecase of a frequency halver 10, the output signal TS2 generally has afrequency which corresponds to half of the frequency of the input signalTS1. Thus, in the present case, a second clock signal TS2 is generatedin a simple manner, the frequency of which corresponds to the frequencyof the interrogation signal AS.

Preferably, the transponder 1 is at least partially implemented as aCMOS-integrated circuit. An integrated circuit is an integratedcircuitry, which means that an electronic circuit comprising a pluralityof electronic components and associated wiring is formed on a commonsubstrate, also called chip. For this, a fully integrated constructioncan be provided in which all electronic components of the transponder 1are arranged on exactly one substrate.

Furthermore, the transponder 1 according to the invention can beproduced using CMOS technology, which means that PMOS transistors, alsocalled p-channel metal-oxide-semiconductor transistors, as well as NMOStransistors, also called n-channel metal-oxide-semiconductortransistors, can be arranged on a common substrate.

The transponder 3 according to the invention enables in particular thegeneration of a time base signal SG, T1, T2 by means of a plurality ofcoils L1, L2, L3 from an amplitude-modulated interrogation signal AS,the amplitude of which is changed in order to transmit different values,since the sum signal SG can be reliably generated even if the amplitudeof the interrogation signals AS is very small due to the modulation.

Reference List

-   1 Transponder system-   2 Interrogator-   3 Transponder-   4.1 First limiter and amplifier-   4.2 Second limiter and amplifier-   4.3 Third limiter and amplifier-   5.1 First voltage-to-current converter-   5.2 Second voltage-to-current converter-   5.3 Third voltage-to-current converter-   6.1 First full-wave rectifier-   6.2 Second full-wave rectifier-   6.3 Third full-wave rectifier-   7 Summing element-   8 Comparator-   9 Low pass-   10 Frequency divider-   L Coil of the interrogator-   P Connections of the coil of the interrogator-   AS Interrogation signal-   f0 Frequency of the interrogation signal-   L1 First coil of the transponder-   S1 Connections of the first coil of the transponder-   A1 Axis of the first coil of the transponder-   E1 First receive signal-   GE1 Rectified first receive signal-   L2 Second coil of the transponder-   S2 Connections of the second coil of the transponder-   A2 Axis of the second coil of the transponder-   GE2 Rectified second receive signal-   L3 Third coil of the transponder-   S3 Connections of the third coil of the transponder-   A3 Axis of the third coil of the transponder-   GE3 Rectified third receive signal-   SG Pulsating sum signal-   RS Reference signal-   TS1 First binary clock signal-   TS2 Second binary clock signal

What is claimed is:
 1. A transponder comprising: a first antenna,coupled to receive an interrogation signal, the first antenna generatesa first electrical signal from the interrogation signal; a secondantenna, coupled to receive the interrogation signal, the second antennagenerates a second electrical signal from the interrogation signal,wherein a first axis of the first antenna and a second axis of thesecond antenna are aligned differently; and a summing element, coupledto receive at least a portion of the first and second electricalsignals, the summing element generates a pulsating sum signal from theat least a portion of the first and second electrical signals, thefrequency of the pulsating sum signal being different from the frequencyof the interrogation signal.
 2. The transponder of claim 1 furthercomprising: a plurality of full-wave rectifiers, coupled to the firstantenna and the second antenna, the plurality of full-wave rectifiersrectifying the first and second electrical signals and providingabsolute values the rectified first and second electrical signals to thesignal summer.
 3. The transponder of claim 1 wherein the frequency ofthe pulsating sum signal is greater than the frequency of theinterrogation signal.
 4. The transponder of claim 3 wherein thefrequency of the pulsating sum signal is twice the frequency of theinterrogation signal.
 5. The transponder of claim 1 further comprising:a third antenna, coupled to receive the interrogation signal, the thirdantenna generates a third electrical signal from the interrogationsignal, the axes of the first, second and third antennae are aligneddifferently.
 6. The transponder of claim 5 wherein the axes of thefirst, second and third antennae are perpendicular to each other.
 7. Thetransponder of claim 1 wherein a comparator is coupled to the summingelement and compares the pulsating sum signal with a reference signal inorder to generate a first clock signal, the frequency of the first clocksignal being a multiple of the frequency of the interrogation signal. 8.The transponder according to claim 7 wherein the multiple is two.
 9. Thetransponder of claim 7 further comprising: at least one low pass filtercoupled to receive at least a portion of the pulsating sum signal, theat least one low pass filter generates a filtered pulsating sum signalthat functions as the reference signal provided to the comparator.
 10. Amethod for generating a time base signal related from an interrogationsignal, the method comprising: generating a first electrical signalderived from the interrogation signal received at a first antenna, thefirst antenna having a first axis; generating a second electrical signalderived from the interrogation signal received at a second antenna, thesecond antenna having a second axis aligned differently than the firstaxis; rectifying the first and second electrical signals; and summingthe rectified first and second electrical signal, and generating apulsating sum signal having a frequency that is different from thefrequency of the interrogation signal by a factor.
 11. The method ofclaim 10 further comprising the steps of: generating a third electricalsignal derived from the interrogation signal received at a thirdantenna, the third antenna having a third axis aligned differently thanthe first and second axes; rectifying the third electrical signal; andsumming the rectified third electrical signal with the first and secondelectrical signals, wherein the pulsating sum signal is partiallyderived from the rectified third electrical signal.
 12. The method ofclaim 11 wherein the axes of the first, second and third antennae areperpendicular to each other.
 13. The method of claim 10 wherein thefactor is an integer greater or less than one.
 14. The method of claim10 further comprising the step of: comparing the pulsating sum signalwith a reference sign to generate a first clock signal, a clockfrequency of the first clock signal corresponding to the frequency ofthe pulsating sum signal.
 15. The method of claim 14 further comprisingthe step of: filtering at least a portion of the pulsating sum signal,the filtered pulsating sum signal functioning as the reference signalthat is used in generating first clock signal.
 16. The method of claim14 further comprising the step of: providing the first clock signal to afrequency divider to generate a second clock signal with a lowerfrequency, the second clock signal being equal to the frequency of theinterrogation signal.
 17. A transponder comprising: a first antenna,coupled to receive the interrogation signal, the first antenna generatesa first electrical signal from the interrogation signal; a secondantenna, coupled to receive the interrogation signal, the second antennagenerates a second electrical signal from the interrogation signal; athird antenna, coupled to receive the interrogation signal, the thirdantenna generates a third electrical signal from the interrogationsignal, the axes of the first, second and third antennae are aligneddifferently; a first full-wave rectifier coupled to the first antenna,the first full-wave rectifier rectifying the first electrical signal; asecond full-wave rectifier coupled to the second antenna, the secondfull-wave rectifier rectifying the second electrical signal; a thirdfull-wave rectifier coupled to the third antenna, the third full-waverectifier rectifying the third electrical signal; a summing element,coupled to receive at least a portion of the first, second and thirdrectified electrical signals, the summing element generates a pulsatingsum signal having a frequency different from the frequency of theinterrogation signal by a factor; a comparator coupled to the summingelement, the comparator compares the pulsating sum signal with areference signal in order to generate a first clock signal, a frequencyof the first clock signal corresponding to the frequency of thepulsating sum signal.
 18. The transponder of claim 17 wherein the factoris an integer.
 19. The transponder of claim 17 wherein the axes of thefirst, second and third antennae are perpendicular to each other.